1. Field of the Invention
This invention relates to a method and apparatus for coating thixotropic high viscosity liquid coating compositions on a moving web at a narrow width through an extrusion type hopper and particularly to a method and apparatus for use in applying a magnetic stripe such as a magnetic sound-recording track to a motion picture film. 2. Description of the Prior Art
In the event a magnetic stripe such as a magnetic sound-recording track is applied to a 16 mm or 8 mm motion picture film, the coating thickness must be uniform along the length direction of the stripe, the coating film must not form a zigzag shape, and the coating thickness along the width direction of the web must be uniform, that is, the sectional shape along the width of the film must have a flat surface as shown in FIG. 1 (a). These conditions are necessary for improving the characteristics of the sound quality by enlarging the contact area with a sound-recording head at the time of recording and reproduction.
The formation of a coating using a known device, as disclosed in U.S. Pat, No. 3,062,181 is shown in FIG. 2. In this device, a magnetic coating liquid supplied from a liquid supply pipe 3 is coated by discharging the liquid from a single discharge orifice 5 formed in an extrustion type hopper 4 to a strip of a web 2 moving in the direction of the arrow around a rotating backing roll 1.
In the above described device, however, it is extremely difficult to obtain a coating film having a flat surface as shown in FIG. 1 (a) in the case of application to narrow coating widths and particularly in the case of stripe application for use with a 8 mm motion picture film or an electronic video recording, as described in, for example, U.S. Pat. Nos. 3,354,264, 3,151,215, etc., film in which the width is about 0.7 mm. The narrower the coating film, the greater the surface tension, with the result that the sectional shape of the coating film becomes a semi-circular shape as shown in FIG. 1 (b). The use of the web after coating without any treatment thereof results in a decrease in the contact area of the recording head and a reduction in the sound quality characteristics, and to scattering of magnetic power chipped by a recording head into a recording machine. In such a case as described, therefore, the coated strip is subjected to a surface treatment such as a calender treatment after coating is completed. However, the surface treatment as described is quite cumbersome, results in a loss of magnetic material and is extremely uneconomical due to the necessity of dust disposal.